let arr: ReadonlyArray<string> = [];

let arr2: readonly string[] = [];

function doStuff(values: ReadonlyArray<string>) {
    const copy = values.slice();
    console.log(`the first value is ${values[0]}`);

    // ...but we can't mutate 'values'
    // values.push('hello'); // 类型“readonly string[]”上不存在属性“push”。
}